The invention relates to a dialysis or diafiltration method. More specifically, the invention relates to the treatment of blood wash fluids or blood fluids to remove urea. Patients suffering from kidney disorders are treated by so-called "blood washing" techniques in order to remove the urea from the blood, which is the largest constituent of the metabolic product of the human protein metabolism. This blood purification takes place by dialysis or diafiltration using semipermeable membranes. By the dialysis technique, the urea is filtered out of the blood through the membrane into a washing fluid; whereas in the diafiltration technique, blood fluids containing the dissolved urea, excluding proteins and blood corpuscles, filter through the membrane. The object of both methods is to obtain a closed circulatory system in which, in the dialysis method, the cleansing fluid is reused after removal of the urea, and with the diafiltration method, the blood fluid is reintroduced into the bloodstream after removal of the urea. A closed circulatory system of the cleansing fluid during dialysis is required to keep the size of the artificial kidneys small.
The pysiological conditions that must be observed when dealing with blood cleansing fluids or blood fluids make it a difficult task to eliminate urea from aqueous solutions. Several methods used to remove urea from cleansing fluid or blood fluids in a closed circulatory system have been investigated, but none has proved satisfactory so far.
An early attempt involved the removal of urea from the cleansing fluid of a dialyzer by using activated charcoal. At room temperature, however, activated carbon adsorbs only about 2 to 3 grams of urea per kilogram; whereas the body must excrete approximately 30 grams during one dialysis procedure. Therefore, about 10 to 15 kilograms of adsorption charcoal would be required. This large quantity, in turn, would require a large apparatus, and the advantages of a reduction of the size of the artificial kidney itself would be lost.
In another known procedure, urease is used to crack the urea enzymatically to ammonia, and the ammonium cations are then adsorbed in a cation exchanger. This procedure has the disadvantage that the pH and the electrolyte balance in the organism are disturbed by the ion exchange. Thus, if the procedure is carried out with blood fluids or recirculated blood cleansing fluid, an additional reinfusion of calcium, potassium, and magnesium ions is required. Furthermore, the sensitivity of the enzyme used for cracking the urea prohibits sterilization of the ion exchanger column. The fact that urease possesses a varying cracking quality, resulting in a non-reproducible cleansing efficiency, presents a further disadvantage.
Additional known procedures use oxystarch, oxycellulose or polyacrolein for conversion with urea. The disadvantage of this method is that these substances react with urea only relatively slowly under physiological conditions, thus requiring too much time for dialysis or diafiltration with a closed circulatory system. The use of oxypolysaccharides presents the problem of slow depolymerization.
Finally, oxidization of urea with nitrates or hypochlorites to remove urea from blood cleansing fluids was proposed. This method, however, presents the danger of the formation of carcinogenic nitrosamines or N-chloroamines.
Thus, it is highly desirable to create a procedure in which urea can be removed from blood cleansing fluids or blood fluids using a solid, which eliminates the above-mentioned disadvantages in the state of the art, achieves the most rapid and complete elimination of urea possible, keeps the amount of solid required for removal at a minimum, and finally, does not cause any reaction products hazardous to the organism.